Biological cells contain receptor molecules located on their external membrane. The function of these receptors is to "sense" the cell environment and supply the cell with an input signal about any changes in the environment. In eukaryotic organisms such cell environment is comprised of the neighboring cells and the function of the receptor is to allow cells to communicate with each other directly (the paracrine regulatory system) or indirectly (the endocrine regulatory system) thus achieving harmonized response of a tissue, organ or a whole organism. In prokaryotic cells, the surface localized receptors provide a means for detecting extracellular environment.
Having received such a signal, neurotransmitters, hormones, chemoattractant or chemorepellant substances for example, the surface localized receptors transmit this information about extracellular environment into the cell through specific intracellular pathways in such a way that the cell responds in the specific fashion to accommodate these changes. When there is an altered supply of the external signal molecules or an altered activity of the cell surface molecules, the cell response would be abnormal causing malfunctioning of a tissue or an organ.
In eukaryotic cells, receptor molecules determine the selective response of the cell. Each type of receptor can interact only with a specific set of ligand molecules. For example, adrenergic receptors interact with adrenaline and noradrenaline, cholinergic receptors interact with acetylcholine, serotoninergic receptors interact with 5-hydroxytriptamine, dopamineergic with DOPA and so on. The cells derived from the different tissues invariably express specific sets of tissue receptors. Different types of receptors are connected to different signal transduction pathways. For example, nicotinic cholinergic receptor, upon binding acetylcholine molecule, directly activates sodium channel (Claudio et al., 1987, is incorporated herein by reference). G-protein coupled receptors activate enzymes of second messenger pathways, for example, adenylate cyclase or phospholipase C with subsequent activation of cAMP or phosphoinositide cascades (Divecha and Itvine, 1995, is incorporated herein by reference). Receptor tyrosine kinases activate cascade of MEK/MAPK kinases leading to cell differentiation and proliferation (Marshall, 1995 and Herskowitz, 1995, are incorporated herein by reference)]. Cytokine receptors activate JAK/STAT cascade which in turn can regulate other pathways as well as activate gene transcription (Hill & Treisman, 1995, is incorporated herein by reference).
Together with the receptors, the cell surface membrane carries ion pumps, ion transporters and ion channels. These molecular assemblies work in concert to maintain intracellular ion homeostasis. Any changes in the activity of these systems would cause a shift in the intracellular concentrations of ions and consequently to the cell metabolic response.
Ion pumps act to maintain transmembrane ion gradients utilizing ATP as a source of energy. The examples of the ion pumps are: Na.sup.+ /K.sup.+ -ATPase maintaining transmembrane gradient of sodium and potassium ions, Ca.sup.2+ -ATPase maintaining transmembrane gradient of calcium ions and H.sup.+ -ATPase maintaining transmembrane gradient of protons.
Ion transporters use the electrochemical energy of transmembrane gradients of one ion species to maintain gradients of other ion counterpart. For example, the Na.sup.+ /Ca.sup.2+ -exchanger uses the chemical potential of the sodium gradient directed inward to pump out calcium ions against their chemical potential.
Ion channels, upon activation, allow for the ions to move across the cell membrane in accordance with their electrochemical potential. There are two main types of ion channels: voltage operated and ligand-gated. Voltage operated channels are activated to the open state upon changes in transmembrane electric potential. Sodium channels in the neuronal axon or L-type calcium channels in neuromuscular junctions exemplify this kind of channel. Ligand-gated channels are activated to the open state upon binding a certain ligand with the chemoreceptor part of their molecules. The classical example of ligand-gated channels is nicotinic cholinergic receptor which, at the same time, is the sodium channel.
There are numerous methods for detecting ligand/receptor interaction. The most conventional are methods where the affinity of a receptor to a substance of interest is measured in radioligand binding assays. In these assays, one measures specific binding of a reference radiolabeled ligand molecule in the presence and in the absence of different concentrations of the compound of interest. The characteristic inhibition parameter of the specific binding of the reference radiolabeled ligand with the compound of interest, IC.sub.50, is taken as a measure of the affinity of the receptor to this compound (Weiland & Molinoff, 1981 and Swillens et all., 1995, are incorporated herein by reference). Recent advances in microchip sensor technology made it possible to measure direct interactions of a receptor molecule with a compound of interest in real time. This method allows for determination of both association and dissociation rate constants with subsequent calculation of the affinity parameter (F_gerstam et al., 1992, is incorporated herein by reference). While being very precise and convenient, these methods do not allow to distinguish between agonist and antagonist activity of the compound.
The type of biological activity of the compounds, agonist or antagonist, may be determined in the cell based assays. In the methods described in Harpold & Brust, 1995, which is incorporated herein by reference, cells cotransfected with a receptor gene and reporter gene construct, are used to provide means for identification of agonist and antagonist potential pharmaceutical compounds. These methods are inconvenient because they require very laborious manipulations with gene transfection procedures, are highly time consuming and use artificially modified cells. Besides, to prove that the agonistic effect of a particular compound is connected to the stimulation of a transfected receptor, several control experiments with a positive and negative control cell lines should be performed as well.
Most closely related to the methods of this invention are the methods described in Parce et al., 1994, which is incorporated herein by reference. These prior art methods use natural cells and are based on registering the natural cell responses, such as the rate of metabolic acidification, to the biologically active compounds. The disadvantage of the prior art is low throughput speed, each measurement point taking about three minutes. Another disadvantage of the prior art is the use of cells immobilized on the internal surface of the measuring microflow chamber. This leads to the necessity of using separate silicon sensors, or cover slips, with the cells adherent to them for each concentration point of the agonist or antagonist, for the receptors that undergo desensitization upon binding to the agonist molecule. This results in high variability of the experimental results.
Ionized calcium, unlike other intracellular ion events, e.g. changes in the intracellular concentrations of protons, sodium, magnesium, or potassium, serves as the most common element in different signal transduction pathways of the cells ranging from bacteria to specialized neurons (Clapham, 1995, is incorporated herein by reference). There are two major pools which supply signal transduction pathways in the cell with the calcium ions, extracellular space and the endoplasmic reticulum. There are several mechanisms to introduce small bursts of calcium into cytosol for signal transduction.
Both excitable and nonexcitable cells have on their plasma membrane predominantly two receptor classes, G-protein coupled serpentine receptors (GPCSR) and the receptor tyrosine kinases (RTK), that control calcium entry into cell cytoplasm. Both GPCSR and RTK receptors activate phospholipase C to convert phosphatidylinositol into inositol(1,4,5)-trisphosphate (InsP.sub.3) and diacylglicerol. InsP.sub.3 acts as an intracellular second messenger and activates specialized receptor that spans the endoplasmic reticular membrane. The activation of this receptor triggers release of calcium ions from the endoplasmic reticulum (Berridge, 1993, is incorporated herein by reference). The calcium ions can also enter the cytoplasm of excitable and nonexcitable cell from extracellular environment through specialized voltage-independent Ca.sup.2+ -selective channels triggered by specific ligands. In nonexcitable cells, hyperpolarization of the plasma cell membrane also enhances entry of calcium ions through passive transmembrane diffusion along the electric potential. For example, opening of potassium channels brings the membrane potential to more negative values inside the cell, thus facilitating Ca.sup.2+ entry across the plasma membrane. Excitable cells contain voltage-dependent Ca.sup.2+ channels on their plasma membrane, which, upon membrane depolarization, open for a short period of time and allow inflow of Ca.sup.2+ from external media into cytoplasm. The endoplasmic reticulum membrane as well as plasma membrane of the excitable cells contains InsP.sub.3 receptors and Ca.sup.2+ -sensitive ryanodine receptors (RyR) releasing Ca.sup.2+ from intracellular stores upon membrane receptor triggered phospholipase C activation or depolarization-induced short burst of Ca.sup.2+ entry into cell cytoplasm from extracellular media respectively.
It is well established that G-protein coupled serpentine receptors initiate Ca.sup.2+ mobilization through the activation of phospholipase C.sub..beta. (Stemweis and Smrcka, 1992, is incorporated herein by reference) whereas tyrosine kinase receptors activate phospholipase C.sub..gamma. with subsequent intracellular Ca.sup.2+ mobilization (Berridge & Irvine, 1989, is incorporated herein by reference).
There are many plasma membrane G-protein coupled serpentine receptors, tyrosine kinase growth factor receptors and voltage- and ligand-regulated channels known to initiate intracellular Ca.sup.2+ mobilization.
Ca.sup.2+ plays an essential role in many functional processes of a cell. For example, Ca.sup.2+ affects the cell cycle (Means, 1994, is incorporated herein by reference) and activates specific transcription factors (Sheng et al., 1991, is incorporated herein by reference). Scores of receptors and ion channels use the Ca.sup.2+ signal to initiate events as basic as cell motility, contraction, secretion, division etc.
Increases in cytosolic and, consequently, in nuclear concentration of the Ca.sup.2+ can also be a cell death promoting signal. For example, prolonged increase in free Ca.sup.2+ activates degradation processes in programmed cell death, apoptosis, activates nucleases that cleave DNA and degrade cell chromatin, promotes DNA digestion by direct stimulation of endonucleases, or indirectly by activation of Ca.sup.2+ -dependent proteases, phosphatases and phospholipases, resulting in a loss of chromatin structural integrity (Nicotera et all., 1994, is incorporated herein by reference).
A development of intracellular fluorescent calcium indicators (Grynkiewicz et all., 1985, is incorporated herein by reference) made it possible for intracellular concentration of free calcium to be measured directly in the living cell. Thus the ability to register changes in intracellular calcium concentration provide the means for monitoring effects of different compounds useful in treating various diseases, whose action is thought to be a result of an interaction with membrane receptors and ion channels.
With the advent of combinatorial chemistry approaches to identify pharmacologically useful compounds, it is increasingly evident that there is a need for methods and apparatuses capable of performing automated characterization of pharmacological profiles and corresponding potencies of the compounds in synthesized combinatorial libraries. This would enable the rapid screening of a large number of compounds in the combinatorial library the identification of those compounds which have biological activity, and the characterization of those compounds in terms of potency, affinity and selectivity.
It is an object of this invention to provide methods for screening and the quantitative characterization of potentially pharmacologically effective compounds that specifically interact with and modulate the activity of cell membrane receptors, ion pumps and ion channels using living cells.
It is an additional object of this invention to provide methods capable of characterizing an affinity of the active compounds to the binding sites of the cell.
It is another additional object of this invention to provide methods to distinguish between agonistic and antagonistic activity of the compounds.
It is yet another additional object of this invention to provide methods to determine the nature of the receptor, ion channel or ion pump entity which is sensitive to the active compounds discovered during the screening process.
It is yet another additional object of this invention to provide methods to characterize cell receptor pattern for particular cell source tissue.
It is yet another additional object of this invention to perform each of the above methods on each member of a series of cell types.
It is yet another additional object of the invention to determine the pattern of cell surface receptors expressed in one or more cell types.
It is yet another additional object of the invention to confirm that a test compound influences the activity of a particular receptor.
It is yet an additional object of the invention to determine the activity of a given receptor in a variety of cell types in which it is expressed.
It is a specific object of this invention to provide an apparatus for fulfillment of the objectives above.
It is yet another additional object of this invention to provide an apparatus for fulfillment of each of the objectives above for each member of a series of cell types.
At least some of these and other objectives are addressed by the various embodiments of the invention disclosed herein.